The present invention relates to a system for controlling the air-fuel ratio for an internal combustion engine emission control system having a three-way catalytic converter, and more particularly to a system for controlling the air-fuel ratio to a value approximating the stoichiometric air-fuel ratio so as to effectively operate the three-way catalyst.
Such a system is a feedback control system, in which an O.sub.2 sensor is provided to sense the oxygen content of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture supplied by a carburetor. The control system comprises a comparator for comparing the output signal of the O.sub.2 sensor with a reference value, an integrating circuit having a proportional circuit connected to the comparator, a driving circuit for producing square wave pulses from the output signal of the integrating circuit, and an on-off type electromagnetic valve for correcting the air-fuel ratio of the mixture. The control system operates to determine whether the feedback signal from the O.sub.2 sensor is higher or lower than a predetermined reference value corresponding to the stoichiometric air-fuel ratio for producing an error signal for actuating the on-off electromagnetic valve to thereby control the air-fuel ratio of the mixture.
In the internal combustion engine, the air-fuel ratio of the air-fuel mixture inherently varies due to the delay of the supply of air and fuel. Describing the variation of the air-fuel ratio in detail with reference to the drawings, FIG. 5(a) shows an opening degree of the throttle valve of the engine. If the throttle valve is rapidly opened as shown in the figure, the amount of induced air increases with the increase of the opening degree as shown in (b). However, the amount of air varies with delay Z.sub.1 and Z.sub.2, respectively. Further, the amount of fuel induced in the cylinders of the engine increases with the increase of the air, as shown in (c), with a delay Z.sub.3 due to delay of the operation of the carburetor and for other reasons such as adhesion of the fuel to the wall of the induction passage of the engine. Because of such a delay of the supplying of fuel, the air-fuel mixture is diluted. FIG. 5(d) shows such a lean air-fuel ratio L, which further causes a rich air-fuel ratio R.
FIG. 6 shows variations of variables in the engine provided with the above described feedback system for controlling the air-fuel ratio. Although variation of the throttle valve opening (FIG. 6(e)) and the amount of air (f) is the same as (a) and (b) of FIG. 5, the fuel and the air-fuel ratio are controlled as shown by (g) and (h). However, a considerable air-fuel ratio deviation including lean and rich air-fuel ratio portions L' and R' is induced by overshooting of the feedback control overshoot for the lean air-fuel mixture.
Such a deviation of the air-fuel ratio also occurs during rapid deceleration of the engine.